We present a weak-lensing study of the cluster A520 based on Advanced Camera for Surveys (ACS) data. The excellent data quality provides a mean source density of ~109?arcmin–2, which improves both resolution and significance of the mass reconstruction compared to a previous study based on Wide Field Planetary Camera 2 (WFPC2) images. We take care in removing instrumental effects such as the charge trailing due to radiation damage of the detector and the position-dependent point-spread function. This new ACS analysis confirms the previous claims that a substantial amount of dark mass is present between two luminous subclusters where we observe very little light. The centroid of the dark peak in the current ACS analysis is offset to the southwest by ~1' with respect to the centroid from the WFPC2 analysis. Interestingly, this new centroid is in better agreement with the location where the X-ray emission is strongest, and the mass-to-light ratio estimated with this centroid is much higher (813 ± 78 M ☉/L R☉) than the previous value; the aperture mass with the WFPC2 centroid provides a consistent mass. Although we cannot provide a definite explanation for the dark peak, we discuss a revised scenario, wherein dark matter with a more conventional range (σDM/m DM 1 cm2 g–1) of self-interacting cross-section can lead to the detection of this dark substructure. If supported by detailed numerical simulations, this hypothesis opens up the possibility that the A520 system can be used to establish a lower limit of the self-interacting cross-section of dark matter.
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机译：我们目前基于高级调查相机（ACS）数据对群集A520进行弱镜头研究。出色的数据质量提供了约109？arcmin–2的平均源密度，与之前基于广角行星相机2（WFPC2）图像的研究相比，它提高了质量重建的分辨率和重要性。我们注意消除仪器的影响，例如由于检测器的辐射损坏和位置相关的点扩展功能而导致的电荷拖尾。这项新的ACS分析证实了先前的说法，即在我们观察到非常少的光的两个发光子簇之间存在大量的暗质量。当前ACS分析中暗峰的质心相对于WFPC2分析中的质心向西南偏移〜1'。有趣的是，这个新质心与X射线发射最强的位置更好地吻合，并且该质心估计的质光比比以前的值高得多（813±78 M☉/LR☉）。 ;具有WFPC2重心的光圈质量提供了一致的质量。尽管我们无法为暗峰提供明确的解释，但我们讨论了一种修订方案，其中具有更常规范围（σDM/ m DM <1 cm2 g-1）的自相互作用横截面的暗物质可以导致检测黑暗的子结构。如果得到详细的数值模拟的支持，则该假设打开了A520系统可用于确定暗物质自相互作用截面的下限的可能性。
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